The existing docs seem to assume that the user is familiar with the
history of packaging in the Python ecosystem (or at least know what is
`distutils`).

Since that is not always the case and that `distutils` is in the process
of being adopted by `setuptools`, the documentation should be changed
to minimize mentions to `distutils` and avoid expecting too much
knowledge from the users.

The benefit of this approach is that it can make the docs more
accessible and easier to understand.

Changes:

- Modify landing page to clarify what `setuptools` does (making it more
  clear to understand for beginners).
- Remove mentions to `distutils`, `transition to PEP 517` from
  `userguide/index`.
  - Instead the text is changed to have a more "introductory" tone.
- Remove mentions to `distutils` from the Quickstart.
- Remove `python2` from the intersphinx mapping - it was causing trouble
  redirecting glossary terms to Python2 docs, instead of Python3.
- Modify documentation about development mode to be more aligned with
  current practices (i.e. using `pip install -e .`)
  - In this process all documentation about running `setuptools` commands
    in `distutils` projects was moved to a new file in `docs/deprecated/running_commands.rst`

Unfortunately sphinx do not accept `.rst` files inside folders that are
added to the `html_static_path`.

(Adding `docs/images` to `html_static_path` will be required in a follow
up commit to adopt the `sphinx-favicon` extension)